Lignocellulosic feedstock is a term commonly used to describe plant-derived biomass comprising cellulose, hemicellulose and lignin. Much attention and effort has been applied in recent years to the production of fuels and chemicals, primarily ethanol, from lignocellulosic feedstocks, such as agricultural wastes and forestry wastes, due to their low cost and wide availability.
The first chemical processing step for converting lignocellulosic feedstock to ethanol, or other fermentation products, involves breaking down the fibrous lignocellulosic material to liberate sugar monomers from the feedstock for conversion to a fermentation product in a subsequent step of fermentation.
There are various known methods for producing fermentable sugars from lignocellulosic feedstocks, the most prominent involving an acid or alkali pretreatment followed by hydrolysis of cellulose with cellulase enzymes and β-glucosidase. The purpose of the pretreatment is to increase the cellulose surface area, with limited conversion of the cellulose to glucose. Acid pretreatment typically hydrolyses the hemicellulose component of the feedstock to yield xylose, glucose, galactose, mannose and arabinose and this is thought to improve the accessibility of the cellulose to cellulase enzymes. The cellulase enzymes hydrolyse cellulose to cellobiose which is then hydrolysed to glucose by β-glucosidase.
After production of a stream comprising fermentable sugar from the lignocellulosic feedstock, the sugars are fermented to ethanol or other fermentation products. If glucose is the predominant substrate present, the fermentation is typically carried out with a yeast that converts this sugar and other hexose sugars present to ethanol, although bacteria are also known for such purpose. This conversion can be carried out by a variety of organisms, including Saccharomyces spp. The ethanol is recovered from the fermentation broth, or “beer”, by distillation. A still bottoms stream comprising dissolved residual organic and inorganic components as well as suspended lignin solids remains after distillation.
Utilizing lignocellulosic feedstocks for ethanol production offers an attractive alternative to burning or land-filling them, which is a practice commonly employed in the agriculture sector. Another advantage of these feedstocks is that the lignin byproduct, which remains after the cellulose conversion process, can be used as a fuel to power the process instead of fossil fuels. Several studies have concluded that, when the entire production and consumption cycle is taken into account, the production of ethanol from lignocellulosic feedstocks generates close to zero greenhouse gases.
However, despite the foregoing advantages, there are still hurdles to be overcome in order to make cellulosic ethanol conversion processes more sustainable. Cellulosic ethanol facilities should be built at large scale to be economically viable, but this requires large amounts of feedstock and consequently large amounts of water. When fresh water requirements for the process are high, it is necessary for the plant to be located near a water source, which in turn reduces the options available for plant site selection. Moreover, treatment of the incoming water and handling and disposal of water effluent from the plant is costly. Zero liquid discharge to the environment would be highly desirable.
Water recycle has been suggested as a means to both reduce fresh water requirements and the amount of wastewater that must be disposed of. However, water recycle has its own set of shortcomings that must be addressed to make it economically feasible.
For instance, recycling streams can increase the levels and/or the nature of the inhibitors in the process, thereby negatively impacting the ethanol production process. The pretreated streams generated during an acidic or basic lignocellulosic pretreatment process contain a number of compounds that are inhibitory to the enzymes used for enzymatic hydrolysis and the microorganisms used for ethanol fermentation (collectively referred to herein as “biocatalysts”). These inhibitory compounds may be inorganic or organic, suspended or dissolved, identified or unidentified and may have additive or synergistic impacts on the biocatalysts. Even if the biocatalysts have been acclimatized to the inhibitors present in process streams, further increases in the concentration of these inhibitors, or the introduction of different inhibitors to process streams containing pre-existing inhibitors, may increase the overall stress on the biocatalysts to the point that their performance is severely impacted.
One potent class of inhibitors generated during cellulosic ethanol conversion processes is phenolic compounds. When plant biomass is pretreated in a cellulosic ethanol process prior to enzymatic hydrolysis, simple or oligomeric phenolics and derivatives can be generated from lignin modification and/or degradation. These compounds are a known inhibitor for biomass-converting enzymes. Examples of phenolic compounds which have been demonstrated to be inhibitory to cellulose-degrading enzymes include vanillin, syringaldehyde, trans-cinnamic acid and hydroxybenzoic acid, as well as phenolic hydroxyl groups associated with lignin itself (Enzyme and Microbial Technology, Vol 46, Issues 3-4, pages 170-176; Journal of Biobased Materials and Bioenergy, Vol 2, No 1, pages 25-32). As these compounds inhibit the enzymes even at very low concentrations, any increase in the concentration, such as through recycle of streams containing these compounds in the process, would further impact the performance of the enzymes.
Another particularly potent inhibitor in cellulosic ethanol conversion processes is acetic acid, which is produced by the release of acetyl groups present on lignocellulosic feedstocks during chemical pretreatment. In particular, acetic acid is a known inhibitor of fermenting microorganisms employed to ferment glucose to ethanol. The microorganism is already inhibited by the natural levels of the acetic acid in the fermentation process, and any further increase in its levels, such as would be the case if streams which contain acetic acid were recycled in the process, would further impact the performance of the microorganism. Other potential inhibitors of biocatalysts generated during pretreatment include inorganic salts, hydroxymethylfurfural (HMF) and furfural.
Acetic acid poses an even larger challenge for cellulosic ethanol processes relative to conventional first generation ethanol processes (i.e., ethanol produced from corn not lignocellulosic feedstock), as the levels present in the streams are much higher. Kellsall and Lyons (“The Alcohol Textbook”, Ed. K. Jaeques, T. P. Lyons and D. R. Kelsall, 1999, Nottingham University Press, Nottingham, United Kingdom, incorporated herein by reference) note that typical levels of acetic acid in a conventional corn ethanol fermentation range between 0.014 and 0.02 wt %. They further note that acetic acid can be produced by contaminants, and that levels at or above 0.05 wt % are known to be inhibitory to yeast. By contrast, in a cellulosic ethanol process, depending on the pretreatment conditions and the composition of the feedstock, acetic acid levels in the feed stream to fermentation can range from 0.1-1.2 wt %, which is between 6 and 70 times more concentrated than corn ethanol processes and above known inhibition levels. Cellulosic conversion processes are also susceptible to bacterial contamination, which can add more acetic acid due to production by the contaminating bacteria. With acetic acid levels already well above typical levels, any further acetic acid added through a recycle process would be detrimental to the process, and would require additional processing to manage, which would impact the economic viability of the process.
Washing steps after pretreatment can help reduce the levels of acetic acid, however adding more water to the process can impact the economics of the process, as the added water must later be removed. Another method that has been proposed to reduce the concentration of inhibitors is a process known as overliming, which involves the addition of lime to precipitate the inhibitors. However, the addition of lime produces gypsum, which is costly to dispose of, results in scale deposition, requires additional water usage and reduces sugar yield.
Anaerobic fermentation of still bottoms remaining after distillation, followed by re-circulation of effluent to the process is one of many potential options the inventors have identified for treating and recycling streams in cellulosic ethanol processes and can be a cost-effective option. However, anaerobic treatment systems are sensitive to the presence of high sulfate levels. In cellulosic conversion processes employing sulfuric acid pretreatment, sulfate salts are generated during adjustment of the pretreated feedstock with alkali. Alternatively, alkali pretreated feedstock can be treated with sulfuric acid prior to enzymatic hydrolysis, which also generates sulfate salts. Regardless of their source, if sulfate-rich streams are sent to anaerobic digesters prior to their recycle, complicated process steps are required to remove these salts prior to anaerobic treatment, which, in turn, can increase both the capital and operating costs of the process.
Thus, there is a need in the art for an improved process of water recycle in a cellulosic conversion process that reduces the build-up of inhibitors, while reducing capital and operating costs.